Also, you should check out
ExRx (Exercise Prescription) on the Net. Lots of excellent info there.
You're right, though -- the best way to get into an exercise routine is to just do it. Go to the gym and jog on the treadmill until you feel tired. Lift some light weights and see how that feels. You don't need to know your body type. There are some basic exercises that apply to everyone: bench press, curls, arm raises, squats, et cetera. If you don't know what those are -- doesn't matter! Imitate what you see other experienced people doing. Or you can look them up online. As Nike says, just do it.